Recently, various display devices such as televisions, computer monitors and the like have been manufactured in the form of flat panel displays, such as LCD, PDP and the like, other than the form of conventional cathode-ray tubes (CRT). There are many cases that, owing to various reasons, micro patterns of conductive materials are formed on a surface of such a flat panel display. For example, a surface of a film may be coated with conductive materials in a network-type pattern to shield electromagnetic waves, or a film having a stripe pattern may be formed on a front substrate or a rear substrate of PDP.
An operation of coating a film with a pattern of conductive materials should be necessarily carried out to form these patterns on the film, and one of the processes widely used in the coating operation is a photolithographic process. The photolithographic process is to form a desired pattern on a film by forming a conductive material on the entire surface of the film, applying a photosensitive material on the conductive material, exposing the photosensitive materials to the UV light (exposure) to correspond to a region to be removed or a remainder region, depending on the use of a negative or positive mode, removing the UV-exposed material or the remainder region and etching a region from which the remainder region is removed.
The use of this photolithographic process is desirable since it is possible to form highly fine patterns, but it has disadvantages that it is complicated, the loss of expensive materials are high since the materials are applied to the other region other than a region to be patterned, and a lot of the cost is required to treat liquid waste generated during the development and etching processes. Also, when the exposure, development and etching equipment used in the photolithographic process is manufactured in a large scale, its manufacturing cost is also high, which is a big obstacle to catch up with a recent trend of a large display screen.
Accordingly, printing processes that use inexpensive equipment and have low material waste and no liquid waste have been increasingly in the limelight. Among them, the most widely used printing processes include a screen printing process, an off-set printing process, and a roll printing process such as a soft roll or hard roll printing of a gravure printing process.
Among them, the screen printing process is a process of forming a pattern by putting a screen engraved with a desired printing pattern on each of films and applying a paste to the film using a roll, etc., but it has a problem that its productivity is extremely low since the pastes is printed on the films one by one.
The off-set printing process is a process of forming a pattern by applying a paste to a hard roll plate that is engraved with a certain pattern and made of glass or metals, filling a groove region of the pattern with the paste using a doctor blade, pressing a soft roll (so-called a blanket), which is composed of silicon rubber and the like, on the hard roll plate to primarily transfer the pattern to the hard roll plate, and pressing the soft roll on a substrate and rolling the soft roll to secondarily transfer the pattern to the substrate.
Also, the soft roll printing process of the gravure printing process is a process of dipping some part of a soft printing roll, which has a groove concavely engraved for printing and is made of materials such as silicon rubber and the like, in a container containing an ink (herein, a paste), or soaking a surface of the roll in a paste in another process, for example, in a process of supplying a paste from the top, removing the ink, which is attached to a region rather than the concave groove, with a doctor blade so that the ink can remain in the concave groove, passing a film between the printing roll and a metallic backup roll facing the printing roll to transfer the remaining paste in the concave groove of the printing roll to the film.
The above-mentioned soft-roll printing process has an advantage that its printability is excellent due to the releasing property and rubber properties in a silicon surface, but joints are inevitably formed in the roll since a soft sheet is processed into a roll shape, and therefore it is impossible to form a desire pattern in the joint region. As a result, defects may occur in the joint region of a sheet when the sheet is rolled at least one time to transfer a pattern to the sheet. Therefore, a roll should be rolled at least one time when a printing process is carried out using a soft roll having the joint region, and therefore length of a region to be printed should be limited to a length range that is less than the circumference of a printing roll. Furthermore, a roll should be changed in size according to the final standard requirements, for example increasing a diameter of a printing roll to print a pattern in a large area, and therefore the equipment used in the soft-roll printing process should be modified and manufactured in a large scale.
In addition, the use of a soft roll such as silicon rubber results in swelling a silicon roll due to the presence of organic solvents mixed in a paste. As a result, when the printing process is repeated several hundred or more times, the silicon roll may be deformed by the swelling of the silicon roll, which leads to the deteriorated pattern accuracy.
As described above, an alternative to the soft roll printing method may be a hard roll printing method where a pattern is printed using a hard roll made of metal, especially stainless steel other than the soft roll made of silicon rubber. There in no limitation on the size of a region to be printed since the hard roll may be manufactured in the perfect shape of cylinder without any of joints, and thus it is possible to solve the problem regarding that rolls of various sizes are prepared according to the size of a region to be printed. Also, the prepared sheet has excellent durability since a roll is not swell by the organic solvents in the paste.
However, when the substrate on which a pattern is printed is so hard (e.g., a glass substrate), the hard roll has a poor releasing property and low rubber characteristics. Therefore, the hard roll has problems that it is impossible to transfer a pattern to the hard substrate, and only the materials such as plastic film are used as the substrate. Polyethylene terephthalate (PET) is widely used as the plastic film, but it has disadvantages that a paste is not suitably transferred from a hard roll since PET has a relatively high surface hardness.